Re: [PATCH] xenpv: don't BUG when failing to setup NMI callback

[Boris Ostrovsky]

On 06/13/2014 08:45 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jun 13, 2014 at 01:26:28PM +0200, Vitaly Kuznetsov wrote:
> > some old Xen hypervisors (prior to 3.2) forbid DomUs to register
> > NMI callbacks. E.g. we have the following code in xen-3.1:
> >
> >      if ( (d->domain_id != 0) || (v->vcpu_id != 0) )
> >          return -EINVAL;
> >
> > Commit 6efa20e49b9cb1db1ab66870cc37323474a75a13 introduced kernel
> > crash in case PV guest fails to register NMI callback. All x86_64
> > PV guests will fail to boot on top of such hypervisors (RHEL5
> > example):
> >
> > (XEN) traps.c:405:d7 Unhandled invalid opcode fault/trap [#6] in domain 7 on VCPU 0 [ec=0000]
> > (XEN) domain_crash_sync called from entry.S
> > (XEN) Domain 7 (vcpu#0) crashed on cpu#3:
> > (XEN) ----[ Xen-3.1.2-389.el5  x86_64  debug=n  Not tainted ]----
> > (XEN) CPU:    3
> > (XEN) RIP:    e033:[<ffffffff81004d96>]
> > (XEN) RFLAGS: 0000000000000282   CONTEXT: guest
> > (XEN) rax: ffffffffffffffea   rbx: 0000000000000000   rcx: 0000000000000002
> > (XEN) rdx: 0000000000000001   rsi: ffffffff81b0fe28   rdi: 0000000000000000
> > (XEN) rbp: ffffffff81b0fe40   rsp: ffffffff81b0fde8   r8:  0000000000000000
> > (XEN) r9:  ffffffff81b0fdd0   r10: 0000000000007ff0   r11: 00000000ffffffff
> > (XEN) r12: ffffffff81d65900   r13: 0000000000000000   r14: 0000000000000000
> > (XEN) r15: 0000000000000000   cr0: 0000000080050033   cr4: 00000000000026b0
> > (XEN) cr3: 000000013a263000   cr2: 0000000000000000
> > (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
> > ...
> >
> > However it is possible to proceed without NMI callback registered.
> > Change BUG() with warning in case of -EINVAL.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Oh, we had a similar patch - somebody reported it earlier - and we
> just checked the version of Xen:
>
> http://lists.xenproject.org/archives/html/xen-devel/2014-05/msg01474.html
>
> But I can't remember why I didn't post it.
>
> However I do like your path of checking the 'ret'.


Should we be checking both ret and version? NMI callback registration 
can return -EINVAL for other reasons (non-canonical address, for example).

-boris


> Vitaly, could expand your patch to also do a check in cvt_gate_to_trap
> so that it won't enable the NMI handler and then lets pick your
> patch?
>
> > ---
> >   arch/x86/xen/setup.c | 11 ++++++++++-
> >   1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> > index 821a11a..5b8b180 100644
> > --- a/arch/x86/xen/setup.c
> > +++ b/arch/x86/xen/setup.c
> > @@ -593,8 +593,17 @@ void xen_enable_syscall(void)
> >   void xen_enable_nmi(void)
> >   {
> >   #ifdef CONFIG_X86_64
> > -	if (register_callback(CALLBACKTYPE_nmi, (char *)nmi))
> > +	int ret;
> > +
> > +	ret = register_callback(CALLBACKTYPE_nmi, (char *)nmi);
> > +	if (ret == -EINVAL) {
> > +		/* Hypervisor probably forbids us to register NMI callback,
> > +		   that is expected when running on top of Xen-3.1 and older */
> > +		pr_warn("xen: failed to register NMI callback\n");
> > +	} else if (ret != 0) {
> > +		/* Other hypervisor failure */
> >   		BUG();
> > +	}
> >   #endif
> >   }
> >   void __init xen_pvmmu_arch_setup(void)
> > --
> > 1.9.3

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